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digitalmars.D.learn - Dynamic multidimensional arrays

reply Bellum <viautbellum gmail.com> writes:
Can anyone point me in the right direction for doing something like this 
in D:

     char[][] anArray;
     int rows, cols;

     ...

     anArray = new char[rows][cols];

It isn't possible in this way because "rows cannot be read at compile 
time", which seems to me to be the point of dynamic arrays. :P
Jul 04 2011
next sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On 2011-07-04 23:42, Bellum wrote:

 Can anyone point me in the right direction for doing something like this
 in D:
 
      char[][] anArray;
      int rows, cols;
 
      ...
 
      anArray = new char[rows][cols];
 
 It isn't possible in this way because "rows cannot be read at compile
 time", which seems to me to be the point of dynamic arrays. :P


auto anArray = new char[][](rows, cols);

Putting the numbers directly in the brackets tries to create a static array 
once you get beyond the first dimension. So,

auto anArary = new char[4][5];

would create a dynamic array of length for with elements which are static 
arrays of length 5. If you want it to by dynamic all the way, you need to put 
the dimensions in the parens like above. Personally, I _never_ put them in the 
brackets, even when the dynamic array has just one dimension. It's just 
simpler to always put them in the parens and not worry about it.

- Jonathan M Davis
Jul 04 2011
parent reply Trass3r <un known.com> writes:
 If you want it to by dynamic all the way, you need to put
 the dimensions in the parens like above. Personally, I _never_ put them  
 in the brackets, even when the dynamic array has just one dimension.  
 It's just
 simpler to always put them in the parens and not worry about it.


Maybe we should force the parens approach even for one dimension.
At least dmd should give better diagnostics. It's damn hard to even find  
that syntax in the docs.
Jul 05 2011
parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On 2011-07-05 11:29, Trass3r wrote:

 If you want it to by dynamic all the way, you need to put
 the dimensions in the parens like above. Personally, I _never_ put them
 in the brackets, even when the dynamic array has just one dimension.
 It's just
 simpler to always put them in the parens and not worry about it.


 
 Maybe we should force the parens approach even for one dimension.
 At least dmd should give better diagnostics. It's damn hard to even find
 that syntax in the docs.


The problem is that there's nothing wrong with putting the dimensions in the 
brackets. It makes perfect sense _if you want dynamic arrays of static 
arrays_. However, that's not normally what people want. _Everyone_ who 
attempts to create a multidimensional dynamic array runs into this problem.

Putting the dimension in the brackets for a single dimension dynamic array 
matches exactly what's going on when you put the dimensions in the brackets 
with multi-dimension arrays. So, disallowing it wouldn't really make sense.

The issue is that it having multi-dimensional arrays where elements are static 
arrays makes perfect sense under certain circumstances, and it uses pretty 
much exactly the syntax that you'd expect it to if you were looking for that, 
but that that syntax is exactly what people try to use when using multi-
dimensional arrays which are purely dynamic. So, this screws over everyone, 
but changing it risks removing a valid feature.

So, I really don't know how it should be handled. I fear that it's like how

int[5][4] a;
int a[4][5];

produce the same array. It matches perfectly how the language works in general 
but is exactly the opposite of what programmers expect, since they don't think 
like a compiler. Changing it makes the language inconsistent, but keeping it 
as-is causes problems for everyone using the language. It sucks pretty much no 
matter what you do. Maybe someone can come up with a way to clean things up, 
but as it stands, static arrays tend to screw over programmers until they 
understand their quirks (generally after getting bitten by errors like the OP 
ran into).

The one thing that _is_ clear, however, is that if the online docs are not 
clear about the paren syntax, then they should be updated. And they should 
probably explain the issue of dynamic arrays of static arrays more clearly so 
that those who read the docs will at least have some clue what is going on and 
avoid this issue.

- Jonathan M Davis
Jul 05 2011
prev sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On 2011-07-04 23:56, Jonathan M Davis wrote:

 On 2011-07-04 23:42, Bellum wrote:

 Can anyone point me in the right direction for doing something like this
 
 in D:
      char[][] anArray;
      int rows, cols;
      
      ...
      
      anArray = new char[rows][cols];
 
 It isn't possible in this way because "rows cannot be read at compile
 time", which seems to me to be the point of dynamic arrays. :P


 
 auto anArray = new char[][](rows, cols);
 
 Putting the numbers directly in the brackets tries to create a static array
 once you get beyond the first dimension. So,
 
 auto anArary = new char[4][5];
 
 would create a dynamic array of length for with elements which are static
 arrays of length 5. If you want it to by dynamic all the way, you need to
 put the dimensions in the parens like above. Personally, I _never_ put
 them in the brackets, even when the dynamic array has just one dimension.
 It's just simpler to always put them in the parens and not worry about it.


Correction,

auto anArray = new char[4][5];

would create a dynamic array of length 5 of static arrays with length 4, 
though

auto anArray = new char[][](4, 5);

does create a dynamic array of length 4 of dynamic arrays of length 5. It 
quickly gets confusing when dealing with dimensions and static arrays IMHO.

- Jonathan M Davis
Jul 05 2011
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Jonathan M Davis Wrote:


 It quickly gets confusing when dealing with dimensions and static arrays IMHO.


I agree. I have raised this topic time ago with no results so far. I find it
surprising that Walter & Andrei too don't think of this as confusing :-|

Bye,
bearophile
Jul 05 2011
prev sibling parent Bellum <viautbellum gmail.com> writes:
On 7/5/2011 2:00 AM, Jonathan M Davis wrote:

 On 2011-07-04 23:56, Jonathan M Davis wrote:

 On 2011-07-04 23:42, Bellum wrote:

 Can anyone point me in the right direction for doing something like this

 in D:
       char[][] anArray;
       int rows, cols;

       ...

       anArray = new char[rows][cols];

 It isn't possible in this way because "rows cannot be read at compile
 time", which seems to me to be the point of dynamic arrays. :P


 auto anArray = new char[][](rows, cols);

 Putting the numbers directly in the brackets tries to create a static array
 once you get beyond the first dimension. So,

 auto anArary = new char[4][5];

 would create a dynamic array of length for with elements which are static
 arrays of length 5. If you want it to by dynamic all the way, you need to
 put the dimensions in the parens like above. Personally, I _never_ put
 them in the brackets, even when the dynamic array has just one dimension.
 It's just simpler to always put them in the parens and not worry about it.


 Correction,

 auto anArray = new char[4][5];

 would create a dynamic array of length 5 of static arrays with length 4,
 though

 auto anArray = new char[][](4, 5);

 does create a dynamic array of length 4 of dynamic arrays of length 5. It
 quickly gets confusing when dealing with dimensions and static arrays IMHO.

 - Jonathan M Davis


Wow, that is confusing. I think I'll stick to using parans, too; thanks 
for the tip!
Jul 05 2011